Focusing coil for cathode-ray tubes



April 24, 1951 R. R. PEARCE 2,550,592

FOCUSING COIL FOR CATHODE-RAY TUBES Filed Aug. 18, 1949 INVENTOR RALPH REGINALD PEARCE BY T TORNEY Patented Apr. 24, 1951 FOOUSIN G COIL FOR CATHODE-RAY TUBES Ralph Reginald Pearce, Hampton, England, as-

.signor to Electric & Musical Industries Limited,

Hayes, England, a company organized unde the laws of Great-Britain ApplicationAugust 18, 1949, Serial'No.111,0'07 In Great Britain August 10, 1948 [This invention relates to focussing coils for electron beams.

In cathode ray tubes suchas .are used 'for television purposes it is'necessary to focus a beamof as is possible in a given winding space to provide an adequate magnetic field for focussing the cathode ray beam. Instead of employing metal 9 Claims. '(Cl. 313-84) 5 With a coil madein accordance with the invention the current paths are planar spirals which are all perpendicular to the axis of the coil and there are substantially no components of electrons so as-to form a small spot on, for ex- 5 current parallel to the axis of the coil as is the ample, the fluorescent screen of the "tube. Frecase with solenoids as heretofore proposed; Such quently, such focussing is effected by a multicomponents of current parallel to the axis of the layer solenoid of wire throughwhich a suitable coil are responsible for the aberrations present electric current is passed, the solenoid surroundin the wire-wound solenoids, and'these are elimiing the neck'of the cathode ray tube and having nated by the use of the spiralwinding according an axial length *which is at least half the .minito the invention.

mum diameter of the solenoid. Such solenoids, When a coil in accordance with the invention however, are found to introduce undesired 'aberis used for focussing a very high voltage beam, rations into the focussed beam so that, in gensuch as is used in cathode ray tubes of the proeral, the luminous spot formed on the screen of jection type, a relatively large value of current the tube is not circular njor is it as small as would is required in order to set up a sufficiently large be possible in the absence of aberrations. Frevalue of ampere turns to produce the required quently, in order to increase the efiiciency of "the magnetic held. In some cases, however, it may solenoid an iron sheath is provided which surbemore convenient'to be able to employ a smaller rounds the solenoid and although the current re value of current, and to enable such smaller cur- :quired to focus the beam is reduced by the use rent to be used it is necessary to increase the --of such a sheathynevertheless, the presence of numberof turnsemployed in the coil. However, the sheath is found to increase the aforesaid if the number of turns of a single planar spiral aberrations. When a beam of electrons is refocussing coil in accordance with the invention quired to be focussed in cathode ray tubes where is increased, the'diameter of the coil may become great brightness and high resolution are necesexcessive and, furthermore, the'quantity'of consary, such as in the so-called projection type of ducto'r employed may become uneconomical. cathode ray tube, it is very desirable that aber- To overcome these difiiculties a focussing coil rations of focus be avoided as far as possible If in accordance with a feature of'the invention is great care is exercised in the winding of the soleprovided which comprises'a plurality of coil units, noid to ensure uniformity of layering and the each in the form of a p a Spiral, Said 0011 units prevention of bunching of the conductors, some being arranged side by de and S ab y conimprovement in performance can" be obtained, d 1

but even with these carefully wound solenoids In O jth e Said invention may be clearly some defects in foc still res lt, I understood and readily carried into efiect, it will It is believed that the aforesaid; aberrations :in now be more fully described with reference to the .focussing are dueto the'flow of components of accompanyi r win in W ich: current parallel to thelaxisof the solenoid and I Figure diagrammatically p e s a oath- .accordingly the object of the present invention is Ode y tube Provided With a focussing C011 in to provide an improved focussing coil in which 40 accordance with the inven an axial flow of current is substantially avoided a Figure 2 is difl-gmmmatic p p e w Of so as to reduce the above-mentioned aberrations. a US oil in co dan ewith One embodi- According to the invention there is provided a menu-lithe ti focussing coil for focussing an electron beam and 1318136 3 represents diagrammatically a which has an axial length of at least one-half the 4:5 cussing 0011 in o a e w th other embodiminimum diameter of the coil wherein the conment f the invention Composed of a D ductor forming the coil is wound in the form of lality of coil units. a planar spiral. The coil may be made from F'gure 4 is a pla View O One y f making metal foil which is wound upon a suitable con- @ifii units, Shown in Figure ducting tubular former and with as many turns 3Figure 5 is a cl'oss'sectiomll View taken 310113 the line 5--5 of Figured, and

Figure 6 shows the electrical connections of the coil of Figures 4 and foil, smetallised paper or similar material may be used.

Figure 1 of the drawings illustrates the invention as applied to the focu'ssing of an electron struction beam in a cathode ray tube of the type employed for television reception. The cathode ray tube is indicated by the reference numeral I and comprises a cathode 2, a cathode screen 3, an accelerating electrode 4, a wall anode and a fluorescent screen 6. The electron beam which emanates from the cathode 2 is focussed into a small spot on the screen 6 by means of a focussing coil 1 which surrounds the neck of the tube 1 In order to overcome the disadvantages hereinbefore referred to when employing the conventional type of wire-wound solenoid coils, the coil '1 in accordance with one embodiment of the invention comprises a strip 8 of aluminium foil of 0.001 inch in thickness and about 2 inches wide and is wound in the form of a planar spiral only a few turns of which are shown in Figure 2, al-' though in practice the coil may, for example,

comprise 1500 turns and may be supplied with a current of l ampere. wound on a conducting formers which may consist of alength of copper tubing to which the inner end of the strip 8 is electrically connected. In order to insulate the turns ofthe foil 8 a paper layer In may be employed between adjacent turns although preferably for insulation purposes the surfaces of the aluminium foil 8 are anodised. Instead of employing metal foil, metallised paper or similar material may be employed. In order to provide an adequate degree of iocussing the axial length of the coil should be at least onehalf the minimum diameter of the coil. The use of a coil as shown in Figure 2 reduces the flow of current parallel to the axis of the coil compared withthe case of a wire-wound solenoid having the same axial length and in order to avoid such ran axial flow as far as possible with the con-. "struction of coil shown in Figure 2, current for "energising the coil is supplied thereto by connecting both ends of the former 9 to one terminal -I of a current source ii the other terminal of which is connected to the centre point of the outer end of th'ecoil as indicated in Figure 2.

The temperature rise of a focussing coil made as shown in Figure 2 is very much less than in thecaseof wire-wound solenoids of the same number of ampere turns, since all of the edges of the turns are exposed to air and are able therefore to disseminate heat rapidly. This is an important property since the breakdown voltage of glass falls rapidly with increase of temperature and with existing solenoid coils the glass of the neck of the tube may puncture under the voltages employed in projection types of cathode ray tubes.

As mentioned above, it may in some cases be desirable to employ a small energising current for the coil and in accordance with another embodiment of the invention the use of a smaller current with a reduced diameter of coil can be achieved by employing a construction in which the focussing coil is composed of a plurality of coil units each in the form of a planar spirrl suitably connected together. Such a coil conis illustrated diagrammatically in Figure 3 from which it will be observed that an odd plurality of coil units i2, I3, M, i5, it, H and I8 are provided, these coil units being arranged side by side and axially along the neck of the cathode ray tube, the units being suitably clamped or held together to form a composite 'focussing coil. The adjacent coil units are arranged so that the spiral turns are in opposite directions, that is to say, the turns of one unit are arranged in a clockwise direction and the The aluminium strip 8 is turns of an adjacent unit are arranged in an anti-clockwise direction. In the arrangement shown in Figure 3, seven coil units are employed and current is supplied to the composite coil by connecting the outermost turn of the central coil unit [5 to one terminal of a current source H, the currentthen flowing to the innermost turn of the central unit which innermost turn is connected to the innermost turns of the adjacent units l4 and IS, the outermost turns of such units Hi and i6 being connected to the outermost turns of the units i3 and H, the innermost turns of these latter units being conterminal 'ofthe current source I i.

,i'low of current is'reduced-to a minimum.

Each coil unit shown in Figure 3 may be wound from wire or narrow tape with the turn 'suitably insulated from one another and each coil unit may be self-supporting by securing adjacent turns together by'means of a suitable adhesive. Alternatively, the turns of each coil unit may be secured to a ring of insulating material and, if desired, opposite sides of said ring may each be provided with a coil'unit. The numberof turns employed in each coil unit in conjunction with the number of coil units employed can be readilyarranged to provide adesired focussing field with a current of a-' practical value. The composite coil is' electrically balanced with respect to the central coil unit and is also radially symmetrical.

Figures 4 and 5 of the drawings illustrate a convenient way of making the coil units and Figure 6 illustrates suitable coil connections for the coils shown in Figures l and 5. A ring-i9 of insulating n'laterial'is provided, the ring being composed, for example, of methyl methac'rylate or other suitable plastic and is provided on each side with spiral grcoves2fl and 2l,'one groove being clockwise and the other anti-clockwise, the grooves being then filled with metal as indicated at 22 and 23 for example, copper, byelectrodeposition or by any other suitable method. The innermost turn of each spiral groove 20, 2|

merges into a concentric groove 24, 25 which,

after being filled with metal, provides contact rings 26, 2'! for the inner ends of the adjacent coil units. Ihe outer ends of the spirals merge into small contact areas 23 and 29 or, alternatively, said ends can be connected to a suitable contact eyelet 30 attached to the insulating ring !9. Connection between the inner ends of the spirals on adjacent insulating rings is made through the medium of shims or rings of thin metal 32 as shown in Figure 5) which engage the contact rings 25, 2! when the coil units are clamped together. Insulation between the spirals on adjacent insulating rings 19 is made by means of paper rings 33 or similar material separately applied between adjacent coil units or insulating layers may be applied to the coil units themselves. The coil units on their insulating rings are assembled into a coaxial pile and suitably clamped or held together to form the composite focussing coil. The coil units when made in accordance with Figures and 5 are connected together in the manner shown in Figure 6. In this figurethe reference letters A indicate coil units which spiral in a clockwise direction other suitable manner.

and the reference letter C coil units which spiral in an anti-clockwise direction. The two centre units C spiral in the same direction so that these two coil units effectively constitute a single unit and hence the composite coil comprises an odd plurality of coil units.

In another method of making a coil unit a ring of suitable thin insulating material, such as a phenolic resin, mica, paper or the like, may be metallised on both sides by securing thereto a thin metal foil or by electro-depositingl metal or by providing it with a metal coating in any The required spiral pattern of the coil, together with suitable contact rings or areas may then be painted, printed or photographically reproduced on to the metallised surface and thereafter unrequired metal areas may be removed in the manner described in the specification of United States patent application No. 690,310. The metallic spiral ultimately formed on said insulating material may be covered with a layer of insulating material as above referred to.

Instead of forming a coil unit in the manner just described, the turns of the spiral may be formed by spraying metal on to an insulating surface through a suitable stencil. Alternatively, a layer of metal applied to an insulating support, such as a layer of foil, may be formed into a continuous spiral by a machining operation by causing a suitable tool to cut away the metal foil s0 forming a continuous spiral.

I claim:

1. An electron discharge device having a focussing coil for focussing the electron beam of said device, said coil having an axial length of at least one-half the minimum diameter of the coil and comprising a length of conductor arrangedin the form of a, planar spiral, adjacent turns of the spiral being insulated from one another, and connections between the inner end of said-conductor and the outer end of said conductor for feeding current through said coil.

2. An electron discharge device having a focussing coil for focussing the electron beam ofsaid device, said coil having an axial length of at least onehalf the minimum diameter of the coil and comprising a length of thin fiat conductor having a width equal to the axial length of said coil and arranged in the form of a planar spiral, adjacent turns of the spiral being insulated from one another, and connections between the inner end of said conductor and the outer end of said conductor for feeding current through said coil.

3. An electron discharge device having a focussing coil for focussing the electron beam oi said device, said coil having an axial length of at .least one-half the minimum diameter of the coil and comprising a length of thin fiat aluminhimconductor having an anodised surface and a width equal to the axial length of said coil, said conductor being arranged in the form of a planar spiral with adjacent turns insulated from one another by said anodised surface, and connections between the inner end of said conductor and the outer end of said conductor forfeeding current through said coil. 1

4. An electron discharge device having a focussing coil for focussing the electron beam of said device, said coil having an axial lengthof; at least one-half the minimum diameter Ofitliiefboil and comprising a plurality of coil units arranged side by side, each coil unit comprising a length of conductor arranged in the form of'a planar spiral with adjacent turns of eachjs'piral insulated from one another, and connections between said coil units to reduce an axial flow of current compared with that of a solenoid coil having the same axial length.

5. A focussing coil for focussing an electron beam having an axial length of at least onehalf the minimum diameter of the coil and comprising an odd plurality of coil units, each unit being composed of a conductor wound in the form of a planar spiral with adjacent turns of the spiral insulated from one another, said units being arranged side by side with the turns of conductor in one unit arranged in an opposite direction from that of the turns in an adjacent unit, a connection for feeding currentt'o the outer turn of the central unit, connections for feeding current tothe outer turns of the outermost units, and connections between the inner ends of the turns of intermediate units and theoutermost turns of intermediate units to reduce an axial flow of current compared with that of a solenoid coil having the same axial length.

6. A focus'sing coil for focussing an electron beam, said coil having an axiallength of at least one-half the minimum diametergof the coil and comprising a conducting former, a length of thin fiat conductor having a width equal to the axial length of said coil and wound on said former in the form of a planar spiral, :means for insulating adjacent turns of said spiral from one another, terminals connected to the inner end of said coil at opposite ends of said former, and a terminal connected to the outer end of said coil. 7

7. A focussing coil according to claim 6, said latter terminal being connected to the center point of the outer end of said coil. J

8. A focussing coil for focussing an electron beam, said. coil having an axial lengthiof at least one-half the minimum diameter of the coil and comprising aconclucting former, a length of thin fiat aluminium conductor having an anodized surface and a, width equal to the axial length of said coil and wound onsaid former inthe form of a planar spiral with the innermost turn in electrical contact with said former and with adjacent turns insulated from one another by said anodized surface, terminals connected to the inner end of said coil at opposite ends of said former, and a terminal connected to the outer end of said 0011.

9. A focussing coil according to claim 8, said latter terminal being connected to the center point of the outer end of said coil. 1

RALPH REGINALD PEARCE.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS 

